Pulsed neutron sources are useful in a variety of applications, such as subterranean operations. Examples of subterranean operations include processes for evaluating subterranean formations. Pulsed neutron spectroscopy involves bombarding a formation with high-energy neutrons and measuring the spectrum of gamma rays emitted by the formation. Different nuclei in formation materials emit characteristic gamma rays, allowing for the estimation of formation constituents and properties such as porosity and oil/gas saturation.
Saturation calculation in some types of formations, such as shaly formations, using pulsed neutron instruments has always been challenging because of the issue of shale handling. For example, pulsed neutron log analysis in shaly sands is complicated because minor variations in shale volume can have a large impact on measurements. Such variations can thus significantly compromise the accuracy of pulsed neutron measurements and the corresponding analysis. Historically, an easier treatment has been to use effective porosity and an in-situ wet shale value for log analysis. For example, conventional techniques such as sigma and C/O (carbon/oxygen) techniques have used the log response in shales as a matrix or calibration value for analysis purposes. However, such techniques can suffer because of data limitation since effective porosity is often not available and shale characteristics can change from zone to zone.